Linear Regression

In the 1880s, Francis Galton plotted parents' heights against their children's adult heights and saw a cloud tilting upward — taller parents, taller children. But the cloud didn't tilt as steeply as the diagonal: the children of the very tall tended to be tall, yet closer to average. The line through that cloud let him PREDICT a child's height from a parent's, and he named the whole idea 'regression.' Today that same line predicts grades, prices, lifespans, and sales — anything where two variables move together.